Cyclic alkenes, kinetic

The Study of the mechanism of the reactions indicated that for aromatic carbonyl compounds, the reaction occurs through a triplet excited state of the carbonyl compound, whereas for aliphatic carbonyl compounds through both singlet and triplet excited states of the carbonyl compound. The reaction is stereospecific for aliphatic carbonyl compounds and gives syn adduct. For cyclic alkenes, kinetically controlled endo-isomer is the major product. The regioselectivity of this cycloaddition reaction depends on the stability and steric interactions of the intermediate diradical. In the reaction of benzophenone with isobutene, the major product is derived from the stable diradical. 

For the catalyst system WCU-CsHbAICIs-CzHsOH, Calderon et al. (3, 22, 46) also proposed a kinetic scheme in which one metal atom, as the active center, is involved. According to this scheme, which was applied by Calderon to both acyclic and cyclic alkenes, the product molecules do not leave the complex in pairs. Rather, after each transalkylidenation step an exchange step occurs, in which one coordinated double bond is exchanged for the double bond of an incoming molecule. In this model the decomposition of the complex that is formed in the transalkylidenation step is specified, whereas in the models discussed earlier it is assumed that the decom-plexation steps, or the desorption steps, are kinetically not significant. 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

As mentioned above, we planned to obtain optically pure styrenyl ethers through Zr-catalyzed kinetic resolution [5] subsequent metal-catalyzed rearrangement would afford optically pure chromenes. However, as shown in Scheme 11, the recovered starting material (40) was obtained with <10% ee (at 60% conversion) upon treatment with 10 mol% (,R)-(EBTHI)Zr-binol (3b) and five equivalents of EtMgCl (70°C, THF). We conjectured that, since the (EBT-HI)Zr-catalyzed reaction provides efficient resolution only when asymmetric alkylation occurs at the cyclic alkene site, competitive reaction at the styrenyl terminal olefin renders the resolution process ineffective. Analysis of the H NMR spectrum of the unpurified reaction mixture supported this contention. Indeed, as shown in Scheme 11, catalytic resolution of disubstituted styrene 49... 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Most of the work reported with these complexes has been concerned with kinetic measurements and suggestions of possible mechanisms. The [Ru(HjO)(EDTA)] / aq. HjOj/ascorbate/dioxane system was used for the oxidation of cyclohexanol to cw-l,3-cyclohexanediol and regarded as a model for peroxidase systems kinetic data and rate laws were derived [773], Kinetic data were recorded for the following systems [Ru(Hj0)(EDTA)]702/aq. ascorbate/dioxane/30°C (an analogue of the Udenfriend system cyclohexanol oxidation) [731] [Ru(H20)(EDTA)]70j/water (alkanes and epoxidation of cyclic alkenes - [Ru (0)(EDTA)] may be involved) [774] [Ru(HjO)(EDTA)]702/water-dioxane (epoxidation of styrenes - a metallo-oxetane intermediate was postulated) [775] [Ru(HjO)(EDTA)]7aq. H O /dioxane (ascorbic acid to dehydroascorbic acid and of cyclohexanol to cyclohexanone)... 

The rhodium(II) complex [Rh2(OAc)4] reduces terminal and cyclic alkenes, activated alkenes and alkynes.148 Various polar solvents could be used, but DMF was preferred. Following a kinetic investigation of the hydrogenation of 1-decene, the mechanism shown in equations (32)-(35) was proposed. 

Trisubstituted cyclic alkenes have been kinetically resolved via a chiral dioxirane (4), generated in situ from the ketone and Oxone. A sequential desymmetrization and kinetic resolution of cyclohexa-1,4-dienes has also been achieved. The observed stereochemical results have been rationalized on the basis of a spiro-planar transition state model.93... 

Kinetic and computational studies by Shea and Kim on MCPBA epoxidations of a series of cyclic alkenes including bridgehead alkenes and tra/w-cycloalkenes have shown that the reactivity depends primarily on the strain energy relief in the transition state <92JA3044>. 

The cross coupling between cyclic alkenes 52 and iodobenzene 39a, which leads to the arylated alkenes 53, 54 and 55 depending on the reaction conditions, has been extensively investigated (Scheme 7.12). In a kinetic study [15] of the reaction between 52a and 39a the rate of reaction was gradually accelerated by increasing the pressure from 1 bar to 8 kbar [krei (1 bar) = 1 fejei (2 kbar) = 4 krei (4 kbar) =... 

The resulting nucleophilic alkoxymethyl radical may be trapped by an electron-deficient alkene. Reduction of the adduct radical (3 by DCA radical anion and protonation of the resulting anion, confirmed by deuterium incorporation from methanol-OD, gives the final product (3%). The diastereoselectivity shown has its origin in a preference for protonation, under kinetic control, from the less hindered side. For acyclic alkenes such as methyl 2-cyanocrotonate or dimethyl maleate, free rotation within (395) results in a low cisJrans ratio of 1.8-2.5 1 whereas for cyclic alkenes such as N,3-dimethylmaleimide or 3-methylmaleimide the cisitrans ratio is considerably higher at 86 14. ... 

Stereochemical and kinetic analyses of the Brpnsted acid-catalysed intramolecular hydroamination/deuterioamination of the electronically non-activated cyclic alkene (13) with a neighbouring sulfonamide nucleophile have been found to proceed as an anh-addition (>90%) across the C=C bond to produce (15). No loss of the label was observed by and NMR (nuclear magnetic resonance) spectroscopies and mass spectrometry (MS). The reaction follows the second-order kinetic law rate = 2 [TfOH] [13] with the activation parameters being = 9.1 0.5 kcal moP and = -35 5 cal moP An inverse a-secondary kinetic isotope effect of d/ h = (1-15 0.03), observed for (13) deuteration at C(2), indicates a partial CN bond formation in the transition state (14). The results are consistent with a mechanism involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C(3) position coupled with an intramolecular anti-addition by the sulfonamide group. ... 

Cyclic Alkenes.—The cyclo-octatetraene complexes [M(j -C5HbX -C8H8)] (M = Co or Rh) form unstable solutions in concentrated sulphuric acid. However, in trifluoroacetic acid the stable cationic species [Mfiy -CsHsXCsH,)] are produced. When M = Co, two isomeric forms (12) and (13) are formed upon initial protonation but over a period of two days irreversible isomerization of (12) to (13) occurs. Kinetics of the isomerization of (12) to (13) for M = Rh are first order in (12). The activation parameters found for this process are A/f = 2fi l kcalmol and A5+ = 12 3calK- mol- ... 

When Ni(LL)(Me4-l,4-benzoquinone), where LL = a bidentate cyclic alkene such as cot, cod, nor, or en /o-dicyclopentadiene, reacts with trimethyl phosphite, it is the alkene LL which is replaced first, en route to the product Ni(P OMe 3)4. This first step is bimolecular, producing an intermediate containing unidentate alkene. Some kinetic parameters are reported, particularly for LL = cyclo-octa-1,5-diene. The formation of the cation [NiH(P OEt 3)4]+ in perchloric acid-methanol solution appears to involve direct protonation of the nickel. Activation parameters for this are = 13 1 kcalmoL and AS = — 2 3 cal deg mol . 

Alkenes react with acyl halides or acid anhydrides in the presence of Lewis acid catalysts. The reaction works better with cyclic alkenes than for acyclic ones. A mechanistically significant feature of this reaction is the kinetic preference for formation of unsaturated ketones. Mechanistic studies using acetic anhydride and zinc chloride as the reagent system are consistent with a two-step mechanism in which the location of the double bond in the product is determined by ease of deprotonation of the initial adduct. A related reaction occurs when alkenes react... 

However, when vinylic metalation is desired, competing allylic deprotonation may occur. In general, thermodynamic acidity and the kinetic preference for vinylic deprotonation of cyclic alkenes decrease with increasing ring size." The stable alkane-soluble reagent n-Butyllithium-Potassium t-Butoxide-TMEDA in hexane metalates Ethylene with potassium and effects selec-... 

It has been suggested that the kinetic preference for formation of (3,y-unsaturated ketones results from an intramolecular deprotonation, as shown in the mechanism above.51 The carbonyl-ene and alkene acylation reactions have several similarities. Both reactions occur most effectively in intramolecular circumstances and provide a useful method for ring closure. Although both reactions appear to occur through highly polarized TSs, there is a strong tendency toward specificity in the proton abstraction step. This specificity and other similarities in the reaction are consistent with a cyclic formulation of the mechanism. 

An interesting extension of this reaction is shown in the asymmetric kinetic resolution of cyclic allylic ether 44 under alkene coupling conditions. Use of (R)-12 as the catalyst gives (R)-45 in > 99% ee at 58% conversion. The ethylated product 46 is also formed in the reaction in 94% ee (Eq. 7) [25]. The reaction is effective for six- to eight-membered 3-oxacycloalkenes 47 as well as for a wide variety of alkoxycycloalkenes 48 [27], with some resolution dependency on the ring size of 47 (Fig. 2) [26]. 

Double bond cis-trans isomerization occurs during hydrogenation with a relative rate dependent on structure. The less stable double bond isomerizes to the more stable one, but, of course, kinetics and thermodynamics control the extent of isomerization. In a linear carbon chain, one can expect the cis alkene to isomerize to trans and vice versa if the thermodynamics are favorable. However, in a strained cyclic system, trans will isomerize to cis (Fig. 2.13).117... 

Cycloaddition of the cyclic nitrone derived from proline benzyl ester with alkenes proceeds readily to give isoxazolidines with good regio-and stereoselectivity (Eq. 8.47).68 The reaction favors exo-mode addition. However, certain cycloadditions are reversible and therefore the product distribution may reflect thermodynamic rather than kinetic control. 

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